Methods and circuit arrangements are used in inertial navigation systems which are based on the idea of measuring accelerations to derive therefrom, by integration over a time interval, the course covered by a ship, airplane or space craft. This requires an earth-bound or space-bound coordinate system. Both coordinate systems can be realized by means of gyro-stabilized platforms.
A space-bound coordinate system is used, for example, for space rockets and other missiles which move far from the earth. The accelerations are measured here relative to a platform stabilized in space. The influence of the gravitational acceleration of the earth is taken into account by the computer. Earth-bound vehicles and airplanes, however, require a platform which is so corrected that it remains constantly horizontal and oriented in a fixed direction.
Frequently used in airplanes are inertial navigation systems where two or three gyros arranged in the horizontal plane with their axes perpendicular to each other act as direction-maintaining gyros during the flight. Two or three accelerometers measure the accelerations in two or three directions perpendicular to each other, so that the path components covered in both partial directions are determined in a navigation computer by double integration, taking into account the rotation of the earth and the Coriolis acceleration. In order to keep the system carrier platform arranged in the horizontal plane constantly horizontal and oriented to the grid north, the angular velocities are calculated from the velocity components, taking into account the rotation of the earth, in the three directions of the space with which the gimballed system carrier platforms must be connected, see for example, (Lueger: Lexikon der Technik, vol. 12, keyword: "inertial navigation").
Difficulties are encountered in gimballed horizontal carrier platforms insofar as the interfering horizontal accelerations are added up on long trips or flights which can lead to considerable navigational errors. It has been tried in the past to eliminate these errors by selecting a correspondingly long natural oscillation period of the carrier platform. Minor natural oscillations are acceptable.
A recent, similarly known navigation system overcomes these difficulties in that all gimbal frames and the motors and tracking devices connected therewith are eliminated and the sensors, hence the gyros and accelerometers, are simply secured to the floor of the vehicle. These navigation systems are called strapdown navigation systems. But since the sensors, that is, the gyros and accelerometers, are mechanically rigidly connected with the vehicle, they must work in a wide dynamic range because they follow all movements of the vehicle. For this reason, it is very difficult to obtain accurate measuring data. On the one hand, minute zero point-and resolution errors may be integrated on long flights or trips to considerable navigational errors. On the other hand, however, strong brief deflections as can appear, for example, during the takeoff of airplanes or by wind gusts, must be fully determined.
For the solution of this problem, presently known navigation systems use gyros and accelerometers with electrical restraint, which permit high, brief overloading of the caging coils and their magnetic circuits. In these instruments, the briefly admissible modulation range (ratio of maximum measured value/smallest still measurable value) is of the order of 10.sup.4 to 10.sup.6.
In strap-down navigations systems, the reference system, which is otherwise fixed mechanically in the position of the gimbal frames to each other, must also be determined, in addition to the sensor data. Since all data must be measured to this end, starting from the takeoff, and must be fed into the computer which then determines the position in space by many iteration steps, particularly accurate measuring data are required in these navigation systems.
Aggravating is the fact that analog-digital converters or amplitude-frequency converters are used for feeding the measuring data into the computer. In this way, quantization errors of the scanning, which can be repeated in each scanning cycle and will thus accumulate over the entire navigation time, have to be added to the zero point-and linearity errors.
In order to take into account zero point errors of various units, it is known to reverse the gyro spin direction during the navigation so that the drift of the gyro is likewise reversed and substantially compensated with increasing navigation time. An arrangement is known for determining the zero point error where the gyros and accelerometers are arranged on a reversing table and where the reversing table is rotatable by 180.degree.. There is a risk, however, that the time frame necessary for evaluating the measured value will no longer be maintained during the reversal of the electrical units.
According to the state of the art, so-called delta modulation is frequently used in the measured value converter acting as an analog-digital converter. Here the measured value is first integrated in an integrator until the threshold switch at the output of the integrator responds and a following pulse transmitter releases a unit resetting pulse. A synchronizer arranged between the threshold switch and the pulse transmitter can bring the release time into a time slot frame which is suited to the rest of the periphery of the navigation computer. The unit resetting pulse is stored by a counter at the input of the navigation computer and resets the integrator by one pulse weight at the same time.
In an electrically restrained rate gyro acting as a primary element, a pulse corresponds e.g: to angle increment .DELTA. .alpha. = .intg. .alpha..multidot. dt, where .alpha. is the angle and .alpha. is the differentiated angle with respect to time. In an accelerometer, a pulse would correspond to a velocity increment.
In order to be able to supply the pulses more accurately, the maximum modulation frequency is selected in this delta modulation technique which is not higher than 10 to 100 kHz. If a vehicle undergoes in a measuring axis very small deflections, but alternating over a long period of time, the measured value modulator gives off a pulse only every 10 seconds, for example, with a scaling of 100 kHz for full deflection and a modulation range of 10.sup.6. Such a delayed quantization is undesirable, however, particularly in a strap-down navigation systems, because the vehicle can turn considerably within 10 seconds and thus falsify the correlation between the earth-bound and the vehicle-bound coordination system.